1. Field of the Invention
This invention relates to a portable power unit which generates a single-phase AC power having a commercial frequency or a like frequency.
2. Prior Art
Conventionally, a portable power unit which is a combination of a small-sized engine and a synchronous generator, for instance, is widely used for emergency purposes, outdoor works, leisure time amusement, etc.
In this type of conventional portable power unit, however, the output frequency depends on the rotational speed of the engine. Therefore, in the case of a bipolar generator, to obtain an AC output of 50 Hz (or 60 Hz), the rotational speed of the engine is required to be held at 3000 rpm (or 3600 rpm), i.e. a relatively low rotational speed, which degrades the operating efficiency of the power unit, and further, necessitates designing the generator to be large in size, resulting in an increased total weight of the power unit.
To overcome this inconvenience, a so-called inverter generator has been proposed by the present assignee, e.g. in Japanese Patent Publication (Kokoku) No. 7-67229 and Japanese Laid-Open Patent Publication (Kokai) No. 4-355672, in which the engine is operated at a relatively high rotational speed to obtain a high AC power from the generator, the AC power is once converted to direct current, and then the direct current is converted to alternating current having a commercial frequency by an inverter.
The conventional inverter generator, however, requires provision of two power conversion blocks, i.e. an AC-to-DC conversion block for once converting the AC power to DC power, and a DC-to-AC conversion block for converting the DC power to AC power having a predetermined frequency, as well as a circuit for temporarily storing the DC power. Thus, the use of a lot of expensive power circuit components is necessitated. This makes it difficult to reduce the size of the generator and leads to an increased manufacturing cost.
As a solution to this problem, the present assignee has further proposed in Japanese Laid-Open Patent Publication (Kokai) No. 10-52046 a portable power unit in which a cycloconverter is employed for use with the generator instead of an inverter, to directly convert a high-frequency AC power generated by the generator to AC power having a predetermined frequency such as a commercial frequency.
However, there still remains a problem as described below even after the inverter is replaced by the cycloconverter.
That is, energy stored in the load returns to the power unit under a low power factor load condition. When the cycloconverter is employed for use with the generator, however, this energy is directly regenerated at the main coil of the generator, so that the cycloconverter is brought into an uncontrollable state due to inability of self-arc extinction when an electric current exceeding the capacity of the generator returns to the input of the cycloconverter, i.e. when an electric current returns at a higher voltage than the voltage of generated electric current.